Stock vs Replacement Tire Calculator
Check a new size against the one your car came with. Use the calculator below. Results update as you type.
How the Stock vs Replacement Tire Calculator Works
This calculator works by checking a replacement size against the factory size on diameter, speed and the ±3% rule. It gives a Pass or Fail result.
- Enter the stock size from the door placard, for example 215/60R16 (26.16 in).
- Enter the replacement, for example 225/55R17 (26.74 in).
- Read the change: +2.24%.
- Read the result: Pass, with a true speed of 61.3 mph at an indicated 60 mph.
The Formula
The formula is Change (%) = (Replacement Diameter − Stock Diameter) ÷ Stock Diameter × 100, followed by a ±3% test.
Change (%) = (D_rep − D_stock) ÷ D_stock × 100 True Speed = Indicated × D_rep ÷ D_stock Result = Pass, if |Change| ≤ 3%Worked example: 235/60R16 is 27.10 in. Against a 26.16 in stock tire, the change is +3.61%, so the result is Fail.
Conversion Table
The conversion table checks 10 replacements against a stock 215/60R16. Eight pass and 2 fail.
Both failures (215/65R16 and 235/60R16) are taller tires on the same 16 in rim. Every 17 in and 18 in option in the table passes.
| Replacement | Diameter (in) | vs Stock | True mph at 60 indicated | ±3% Check |
|---|---|---|---|---|
| 205/65R16 | 26.49 | +1.28% | 60.8 | Pass |
| 215/65R16 | 27.00 | +3.24% | 61.9 | Fail |
| 225/60R16 | 26.63 | +1.81% | 61.1 | Pass |
| 205/55R17 | 25.88 | -1.07% | 59.4 | Pass |
| 215/55R17 | 26.31 | +0.59% | 60.4 | Pass |
| 225/50R17 | 25.86 | -1.14% | 59.3 | Pass |
| 225/45R18 | 25.97 | -0.71% | 59.6 | Pass |
| 235/40R18 | 25.40 | -2.89% | 58.3 | Pass |
| 225/55R17 | 26.74 | +2.24% | 61.3 | Pass |
| 235/60R16 | 27.10 | +3.61% | 62.2 | Fail |
What Is the Stock vs Replacement Tire Calculator?
The Stock vs Replacement Tire Calculator is a tool that tests whether a replacement tire size is a safe match for the factory size. It uses the ±3% diameter window that most tire shops apply.
The calculator treats the stock size as fixed. The factory size is the size the speedometer, ABS, traction control and transmission were calibrated for.
Common Uses
This calculator has 4 uses: shop verification, online orders, used-car checks and insurance records.
- Verify shop advice: Check a substitute size a shop suggests.
- Check online orders: Confirm a size before paying.
- Inspect used cars: See whether fitted tires match the placard.
- Document changes: Record the size change for insurance or inspection.